Liquid flow sensors in the form of MEMS devices are configured to measure properties of liquid in contact with the sensors and provide output signals representative of the liquid flow rates. Thermal liquid flow sensors are configured to heat the liquid and measure the resulting thermal properties of the liquids to determine flow rates. Such thermal flow sensors generally include a microsensor die that includes a substrate and one or more elements disposed on the substrate for heating the liquid and sensing the liquid thermal properties. A microbridge sensor, for example as detailed in U.S. Pat. No. 4,651,564 to Johnson et al., is an example of such a thermal flow sensor. The microbridge sensor includes a flow sensor chip which has a thin film bridge structure thermally insulated from the chip substrate.
A pair of temperature sensing resistive elements are arranged on the upper surface of the bridge either side of a heater element such that, when the bridge is immersed in the liquid stream, the flow of the liquid cools the temperature sensing element on the upstream side and promotes heat conduction from the heater element to thereby heat the temperature sensing element on the downstream side. The temperature differential between the upstream and downstream sensing elements, which increases with increasing flow speed, is converted into an output voltage by incorporating the sensing elements in a Wheatstone bridge circuit such that the flow speed of the gas or liquid can be detected by correlating the output voltage with the flow speed. When there is no liquid flow, there is no temperature differential because the upstream and downstream sensing elements are at similar temperatures.
However, microbridge liquid sensors intermittently exhibit drift behavior making the sensor measurements susceptible to error and instability.
Another example of a thermal sensor is a microstructure thermal flow sensor having a microsensor die with a Microbrick® or microfill structure which sensor is more suited to measuring liquid flow and properties under harsh environmental conditions. Note that the term Microbrick® is a registered trademark of Honeywell Inc. of Morristown, N.J. The microstructure flow sensor uses a Microbrick® or micro fill forming a substantially solid structure beneath the heating/sensing elements and has a passivation layer isolating the heating/sensing elements from the fluid so that the sensor is less susceptible to the effects of the fluid. Examples of such microbrick thermal flow sensors are disclosed in U.S. Pat. No. 6,794,981 entitled “Integratable-fluid flow and property microsensor assembly” issued on Sep. 21, 2004, to Padmanabhan et al. Although this type of microstructure sensor is capable of reliable and rapid-response operation under harsh environments, this type of sensor still generally exhibits drift behavior which leads to measurement error.
The aforementioned problems demonstrate that there is a need to provide a liquid flow sensor which is capable of accurately and reliably measuring liquid flow in a stable manner.